1. Field of the Invention
This invention relates to the conversion of "land plaster" or of synthetically produced calcium sulfate dihydrate powder to the relatively insoluble or "dead-burned" calcium sulfate anhydrite; and more particularly to such a process in which the particles being processed undergo a "self-grinding" comminution without the use of extensive mechanically motivated size-reducing means or mechanisms known in this art.
"Land plaster" is a ground, as-mined gypsum and is predominately calcium sulfate dihydrate with some varying small amounts of calcium sulfate anhydrite and other mineral impurities.
"Dead burned" anhydrite, in comparison to soluble anhydrite which will set rapidly when mixed with water, is the anhydrous salt of gypsum which re-hydrates in water very slowly. Due to such characteristic it is emminently suitable for many chemical processing uses such as fillers both in the chemical processing areas of plastics, paints and the like and in the food area provided that the product meets governmental standards for purity. It is well known in the art to roast or calcine the gypsum rock to convert the calcium sulfate dihydrate (CaSO.sub.4.2H.sub.2 O) with heat to calcium sulfate anhydrite (CaSO.sub.4). The generally accepted method of doing this, however, involves the use of "beehive" kilns in which 4 to 12 inch sized rocks are placed and then subjected to calcination by high temperature air circulation (900.degree. to 1250.degree. F) for extremely long periods of time (60-80 hours), followed by extensive grinding of the large sized calcined rock by crushers and impact or attrition mills to produce a uniformly sized extremely fine calcium sulfate anhydrite powder. Such a process is both labor and energy intensive; is a batch process; and requires considerable time for the processing of each batch. Further, many of the resultant end uses of the calcium sulfate anhydrite in insoluble form call for extreme uniformity of color and a minimum of mineral impurities. These additional requirements may necessitate washing and the hand selection of individual pieces of rock for processing. In addition the calcined rock then must be ground, and this necessitates the addition of intermediate product storage facilities; additional apparatus and means to perform the grinding; and further processing time.
2. Description of the Prior Art
In another typical prior art procedure, calcination is accomplished in rotating kilns, either inclined slightly from the horizontal or provided vertically with lifting movable shelves. Such rotary kilns still further require specially-sized feed material and the additional and separate steps of mechanically grinding the resultant product.
In a somewhat related, though distantly, series of endeavors in this field, it is known to continuously calcine crushed gypsum in heated air or hot gases to produce a partial calcination to the "Plaster of Paris" or hemihydrate state (CaSO.sub.4.1/2 H.sub.2 O). This is illustrated in U.S. Pat. No. 1,984,201 disclosing continuously introducing crushed gypsum into a grinding mill to which heated air of hot gases are also introduced. This effects a cocommitant grinding by mechanical means since the calcining apparatus is also and principally a comminution apparatus. U.S. Pat. No. 3,648,994 also discloses calcining gypsum in a current of heating gas to plaster products, but in a cyclone plant wherein the current of heating gas and calcining product is in a vortex stream. This process again is followed by a mechanical, separate and additional grinding step, and also relates to the preparation of water soluble products.
In an unrelated area of endeavor and different field of interest, it is known to "exfoliate" certain inorganic minerals wherein there is an increase in dimension of the individual particles by heat treatment. Representative patents of interest in this area include U.S. Pat. Nos. 2,531,975; 2,572,483; 2,619,776; 2,763,479; and 3,097,832. As being related to this area there should also be mentioned U.S. Pat. No. 3,862,294 which discloses calcining certain inorganic mineral materials to their oxides using co-current contact with a gaseous heat source without any mention of change of particle size.